Supercritical Fluids, Systems and Methods for Use

ABSTRACT

A supercritical fluid comprises carbon dioxide and at least one disorder-inducing species. The proportion of carbon dioxide to the at least one disorder-inducing species in the supercritical fluid may be sufficient to induce disorder in the fluid. Power generation systems and thermal energy storage systems configured to use the supercritical fluid are described.

CROSS-REFERENCE

This application is a continuation application of PCT Application No.PCT/US2012/039217, filed on May 23, 2012, which claims the benefit ofU.S. Provisional Patent Application No. 61/489,605, filed on May 24,2011; this application also claims the benefit of U.S. ProvisionalApplication No. 61/563,802, filed on Nov. 27, 2011, all of which areentirely incorporated herein by reference.

BACKGROUND OF THE INVENTION

There are various types of approaches for thermal energy storage, whichmay be broadly classified under sensible heat storage, latent heatstorage and chemical energy storage. Storage and removal of energy usingsensible heat storage involves a temperature change of the storagemedium in solid, liquid or gaseous form. Storage and removal of energyusing latent heat storage involves a state change of the storage medium,e.g., liquid to gas. Storage and removal of energy using chemical energystorage involves a chemical change in the storage medium, e.g., burninghydrogen.

Some thermal energy technologies rely on sensible heat energy. Phasechange materials, in contrast, rely on the latent heat energy and cantherefore store substantially large amounts of heat. Thermal energytechnologies having phase change materials may require large heattransfer areas.

There are limitations associated with current thermal energy storagesystems. Chemical reactions may offer interesting tunable systems withkey challenges around kinetics and mass transfer for the reversiblereactions. Sensible heat storage materials (e.g. molten salt) may havenarrow temperature range limitations, but are widely used inconcentrating solar. Phase change materials may be limited to lowtemperatures (<200° C.).

As system temperatures increase, costs associated with containmentmaterials may also increase. A key challenge is to minimize containmentmaterial cost.

SUMMARY OF THE INVENTION

In view of the limitations associated with current thermal energystorage systems recognized herein, there is a need in the art forimproved apparatuses and methods for energy storage.

The invention provides energy storage systems that benefit from improvedheat capacities in relation to other systems, while minimizing materialsand operating costs and expenses. Some embodiments provide asupercritical fluid mixture that comprises carbon dioxide. Thesupercritical fluid mixture can include other components, such ashydrocarbons. Provided herein are efficient, high energy density, costeffective, reliable and maintainable thermal energy storage systems.

In some embodiments, supercritical fluid mixtures are provided havingsubstantial increase in heat capacity in relation to other systems. Thesubstantial increase in heat capacity may be on the order of at least1.1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, or 10 over other fluid mixtures. Insome cases, supercritical fluid mixtures provided herein comprising CO₂and one or more secondary components may have heat capacities on theorder of at least 1.1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, or 10 over asupercritical fluid that only includes CO₂.

In some embodiments, supercritical fluids mixtures include two or morecomponents. In some situations, a supercritical fluid includes carbondioxide and other fluids.

In some embodiments, supercritical fluid mixtures are provided with heatcapacities that are targeted to increase over broad temperature ranges.Such supercritical fluid mixtures may be tailored for particularapplications—for example, for concentrating solar power, the mixturesmay be able to store significant energy up to a temperature of about600° C. For geothermal processes, lower temperatures may be acceptable.

In some embodiments, supercritical fluid mixtures are provided withsubstantially high heat capacities, but configured to operate in amanner that does not substantially increase system pressure. Thisadvantageously reduces containment costs, which may decrease overallsystem and operating costs.

In some embodiments, supercritical fluid mixtures are provided for useas working fluids and thermal energy storage mediums in energy storagesystems that operate per a Brayton cycle.

In some embodiments, supercritical fluid mixtures are provided for useas thermal energy storage mediums in energy storage systems that operateper a Rankine cycle.

An aspect of the invention provides a system, comprising a circulatoryfluid flow path having (a) a compressor configured to accept a workingfluid and increase the pressure of the working fluid, wherein theworking fluid is a supercritical fluid mixture comprising carbon dioxideand at least one disorder-inducing species; (b) a heat exchangerdownstream of the compressor, the heat exchanger adapted to accept theworking fluid from the compressor and provide heat to the working fluid;and (c) a power generator downstream of the heat exchanger, the powergenerator adapted to generate power upon the flow of the working fluidthrough the power generator, and direct the working fluid to thecompressor. The system (i) circulates the working fluid along thecirculatory fluid flow path, and (ii) increases the pressure and/ortemperature of the working fluid to above the critical pressure andcritical temperature of the working fluid. In an embodiment, the systemfurther comprises an another circulatory fluid flow path for circulatingan another working fluid, the another fluid flow path comprising: (a) ananother heat exchanger for providing energy to the another workingfluid; (b) a pump downstream of the another heat exchanger, the pumpadapted to accept an another working fluid and increase the pressure ofthe another working fluid; and (c) the heat exchanger. The heatexchanger accepts the another working fluid and transfers energy fromthe another working fluid to the working fluid. In another embodiment,the system further comprises an energy source in thermal communicationwith the another heat exchanger. The energy source provides energy tothe another heat exchanger. In another embodiment, the energy source isa combustion system, nuclear reactor or solar concentrator. In anotherembodiment, the another working fluid comprises a supercritical fluidmixture comprising carbon dioxide and at least one disorder-inducingspecies. In another embodiment, the system further comprises (d) ananother heat exchanger downstream of the power generator, the anotherheat exchanger adapted to (i) accept the working fluid from the powergenerator, (ii) remove heat from the working fluid and (iii) direct theworking fluid to the compressor. In another embodiment, the at least onedisorder-inducing species is selected from the group consisting of analkane, alkene, alcohol, aldehyde, ketone, ether, ester, water,fluorinated hydrocarbons, nitromethane, aromatic hydrocarbons andcarboxylic acid. In another embodiment, the compressor increases thepressure of the working fluid at or above a critical pressure of theworking fluid. In another embodiment, the heat exchanger maintains orincreases the temperature of the working fluid at or above a criticaltemperature of the working fluid.

Another aspect of the invention provides a system comprising acirculatory fluid flow path having: (a) a first heat exchanger adaptedto accept a working fluid and provide energy to the working fluid. Theworking fluid is a supercritical fluid mixture comprising carbon dioxideand at least one disorder-inducing species; (b) a pump downstream of thefirst heat exchanger. The pump accepts the working fluid from the firstheat exchanger and increases the pressure of the working fluid; and (c)a second heat exchanger downstream of the pump, the second heatexchanger adapted to accept the working fluid from the pump and transferenergy from the working fluid to a secondary fluid. The systemcirculates the working fluid along the circulatory fluid flow path. Inan embodiment, the system further comprises an energy source in thermalcommunication with the first heat exchanger. The energy source providesenergy to the first heat exchanger. In another embodiment, the energysource is a combustion system, nuclear reactor of solar concentrator. Inanother embodiment, the system maintains the pressure and temperature ofthe working fluid at a supercritical pressure and supercriticaltemperature, respectively, of the working fluid. In another embodiment,the at least one disorder-inducing species is selected from the groupconsisting of an alkane, alkene, alcohol, aldehyde, ketone, ether,ester, water, fluorinated hydrocarbons, nitromethane, aromatichydrocarbons and carboxylic acid.

Another aspect of the invention provides a thermal energy storage fluid,comprising carbon dioxide and at least one disorder-inducing species.The proportion of carbon dioxide to the at least one disorder-inducingspecies in the thermal energy storage fluid is sufficient to inducedisorder in the thermal energy storage fluid at or above a criticalpoint of the thermal energy storage fluid. In an embodiment, the atleast one disorder-inducing species is selected from the groupconsisting of an alkane, alkene, alcohol, aldehyde, ketone, ether,ester, water, fluorinated hydrocarbons, nitromethane, aromatichydrocarbons and carboxylic acid. In another embodiment, the thermalenergy storage fluid has a heat capacity of at least about 1 Kilojoule(KJ)/Kg*K, 2 KJ/Kg*K, 3 KJ/Kg*K, 4 KJ/Kg*K, 5 KJ/Kg*K, 6 KJ/Kg*K, 7KJ/Kg*K, 8 KJ/Kg*K, 9 KJ/Kg*K, 10 KJ/Kg*K, 15 KJ/Kg*K, 20 KJ/Kg*K, 25KJ/Kg*K, 30 KJ/Kg*K, 40 KJ/Kg*K, or 50 KJ/Kg*K. In another embodiment,the proportion is at least about 0.5:1. In another embodiment, theproportion is at least about 1:1. In another embodiment, the proportionis at least about 2:1.

Another aspect of the invention provides a method for forming a thermalenergy storage fluid, comprising providing a fluid mixture having carbondioxide and at least one disorder-inducing species in a vessel at aproportion of carbon dioxide to the at least one disorder-inducingspecies that is selected to induce disorder in the thermal energystorage fluid at or above a critical point of the thermal energy storagefluid. In an embodiment, the at least one disorder-inducing species isselected from the group consisting of an alkane, alkene, alcohol,aldehyde, ketone, ether, ester, water, fluorinated hydrocarbons,nitromethane, aromatic hydrocarbons and carboxylic acid. In anotherembodiment, the thermal energy storage fluid has a heat capacity of atleast about 1 KJ/Kg*K, 2 KJ/Kg*K, 3 KJ/Kg*K, 4 KJ/Kg*K, 5 KJ/Kg*K, 6KJ/Kg*K, 7 KJ/Kg*K, 8 KJ/Kg*K, 9 KJ/Kg*K, 10 KJ/Kg*K, 15 KJ/Kg*K, 20KJ/Kg*K, 25 KJ/Kg*K, 30 KJ/Kg*K, 40 KJ/Kg*K, or 50 KJ/Kg*K. In anotherembodiment, the proportion is at least about 0.5:1. In anotherembodiment, the proportion is at least about 1:1. In another embodiment,the proportion is at least about 2:1.

Additional aspects and advantages of the present disclosure will becomereadily apparent to those skilled in this art from the followingdetailed description, wherein only illustrative embodiments of thepresent disclosure are shown and described. As will be realized, thepresent disclosure is capable of other and different embodiments, andits several details are capable of modifications in various obviousrespects, all without departing from the disclosure. Accordingly, thedrawings and description are to be regarded as illustrative in nature,and not as restrictive.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in thisspecification are herein incorporated by reference in their entiretiesto the same extent as if each individual publication, patent, or patentapplication was specifically and individually indicated to beincorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe appended claims. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1 shows a phase diagram of carbon dioxide;

FIG. 2 shows a plot of the heat capacity of carbon dioxide alongspecified isotherms (Tr);

FIG. 3 shows a phase diagram of a fluid;

FIG. 4 is a phase diagram of a fluid mixture, showing the critical line;

FIG. 5 is an exemplary plot of heat capacity (Cp) as a function oftemperature (T) and component composition (X) for a fluid mixturecomprising nitromethane and isobutanol;

FIG. 6 is a cross-section of the plot of FIG. 5;

FIG. 7 is a system configured to generate power, in accordance with anembodiment of the invention;

FIGS. 8A and 8B show pressure-volume and temperature-entropy diagramsfor a system operating under a Brayton cycle;

FIG. 9 shows a system configured to generate power, in accordance withan embodiment of the invention;

FIG. 10 shows a system configured to store energy and generate power, inaccordance with an embodiment of the invention; and

FIG. 11 shows a system for storing thermal energy, in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

While various embodiments of the invention have been shown and describedherein, it will be obvious to those skilled in the art that suchembodiments are provided by way of example only. Numerous variations,changes, and substitutions may occur to those skilled in the art withoutdeparting from the invention. It should be understood that variousalternatives to the embodiments of the invention described herein may beemployed in practicing the invention.

The term “fluid” generally refers to a substance that continuallydeforms or flows under an applied shear stress. A fluid can have variousphases, such as solid phase, liquid phase, gas phase, or supercriticalphase. A fluid can include a liquid, gas, plasma, or semi-solid, such asa gel-like substance, or a mixture of fluids, such as a gas-liquidmixture. In some embodiments, a fluid can be selected from an organic(e.g., carbon-containing species) and/or inorganic substances, such as asubstance including one or more —OH groups, ═O groups, carbon-to-carbondouble bonds, and/or carbon-to-carbon triple bonds. In an example, afluid can be selected from water, alcohols (e.g., methanol, ethanol),aldehydes, ketones, carboxylic acids, and combinations thereof, such asa water-alcohol mixture (e.g., water-methanol mixture). A fluid can be afluid mixture having two or more components.

The term “supercritical fluid” generally refers to a substance at atemperature and pressure above its critical point, where distinct liquidand gas phases do not exist. A supercritical fluid can effuse throughsolids like a gas, and dissolve materials in a manner similar to aliquid. In addition, close to the critical point, small changes inpressure or temperature can result in large changes in fluid density.Examples of supercritical fluids include carbon dioxide (CO₂) and water.

The term “secondary fluid,” as used herein, refers to a fluid for use inremoving heat from, or adding heat to, another fluid. A secondary fluidcan be a liquid, gas, gas-solid or gas-liquid mixture. In some cases, asecondary fluid is air.

The term “disorder-inducing species” generally refers to an atom ormolecule, or a mixture of atoms and/or molecules, which is adapted togenerate disorder in a fluid at or above the critical point of thefluid. For example, in a fluid mixture having CO₂ and adisorder-inducing species, the disorder-inducing species can generatedisorder in the fluid mixture at or above the critical point of thefluid mixture. This may be effected, for example, by reduced (ordisrupted) intermolecular interactions between atoms or molecules of thefluid, such as between CO₂ molecules, and in some cases increasedattractive interactions between CO₂ or disorder-inducing species, whichmay result in local inhomogeneities and extended disorder at or abovethe critical point. For example, a disorder-inducing species in a fluidmixture comprising CO₂ can disrupt the interaction between CO₂ moleculesin the fluid mixture. In some embodiments, a disorder-inducing speciesintroduces disorder in a fluid mixture comprising supercritical CO₂. Insome embodiments, the disorder-inducing species is an organic species,such as an alkane, alkene, alcohol, aldehyde, ketone, ether, ester,water, fluorinated hydrocarbons, nitromethane, aromatic hydrocarbons andcarboxylic acid.

The term “cycle,” as used herein, refers to a system having one or morecomponents (or unit operations, also “units” herein) for facilitatingfluid flow and/or fluid phase change, such as pumps, compressors, fluidseparators, heat exchangers and reservoirs (or vessels). A cycle can bea circulatory flow system. In the context of such circulatory flowsystems, the terms “downstream” and “upstream” are used to indicate thelocation of one component in relation to another component along a fluidflow path that brings the components in fluid communication with oneanother. Components can be interconnected with the aid of fluid flowpaths (or fluid streams, also “streams” herein), which can includechannels, fluid passages or conduits for aiding in fluid flow from oneunit to another.

FIG. 1 is a phase diagram of carbon dioxide as a function of pressure(bar) and temperature (Kelvin, K). Below the critical temperature, e.g.,280 K, CO₂ exists as a gas, liquid or solid, depending on the pressure.As the pressure increases, the gas compresses and eventually (at justover 40 bar) condenses into a much denser liquid. The system of FIG. 1includes two phases in equilibrium, a dense liquid and a low densitygas. As the critical temperature is approached (300 K), the density ofthe gas at equilibrium becomes denser, and that of the liquid lower. Atthe critical point, (304.1 K and 7.38 MPa (73.8 bar)), there is nodifference in density, and the two phases of CO₂ become one fluid phase,namely a supercritical fluid. In some situations, for carbon dioxide at400 K, the density increases almost linearly with pressure.

FIG. 2 is a plot of the heat capacity of carbon dioxide along specifiedisotherms (Tr). The invention provides supercritical fluid mixtures thatcan have both broadened and increased heat capacities with minimalsystem pressure increases.

A supercritical fluid may be generated by increasing the pressure of afluid at a constant temperature above the critical temperature;increasing the temperature of the fluid at a constant pressure above thecritical pressure; or increasing the temperature and the pressure of thefluid to a point above the critical temperature and critical pressure.The pressure of the fluid can be increased at constant (or substantiallyconstant) volume.

A fluid may be pressurized with the aid of a compressor or pump. As anexample, at a temperature above the critical temperature, CO₂ may becompressed by a single or multi-stage compressor to a pressure above thecritical pressure of CO₂ to generate supercritical CO₂.

The invention provides fluid mixtures that are based on the unexpectedrealization that by introducing disorder in a supercritical fluidmixture, the thermal energy properties, such as heat capacity, of thefluid mixture can be tailored for use in a given application, such as athermal energy storage application. Such fluid mixtures can be used inenergy storage systems, such as systems adapted for use in a Brayton orRankine Cycle. In some embodiments, an ultra-thermally dense thermalenergy storage fluid is provided by enhancing critical fluctuations andintroducing disorder to tailor the properties of supercritical fluidsover a wide range in pressure (P), temperature (T) and composition (X).Fluid mixtures provided herein can demonstrate extended thermal capacityalong the critical line of the fluid mixtures and throughout theadjacent critical surface.

In some embodiments, a fluid mixture comprises a primary component (atomor molecule) and one or more secondary components. The primary componentcan be a non-carcinogenic and/or an environmentally friendly species,and the one or more secondary components can be disorder-inducingspecies. The secondary components may be non-carcinogenic and/orenvironmentally friendly. In some embodiments, the primary component isselected from CO₂ and H₂O. In an example, the primary component is CO₂.In another example, the primary component is H₂O. In another example,the primary component is a mixture of CO₂ and H₂O. The temperature,pressure and composition of the fluid are selected such that the fluidmixture is supercritical.

Thermal Energy Storage Fluids

An aspect of the invention provides a thermal energy storage fluidcomprising carbon dioxide and at least one disorder-inducing species.The proportion of carbon dioxide to the at least one disorder-inducingspecies in the thermal energy storage fluid is selected such thatdisorder is induced in the thermal energy storage fluid at or above acritical point of the thermal energy storage fluid. In some cases,disorder is induced at or above a critical point of CO₂. The fluid withthe disorder-inducing species can be at a pressure and temperature suchthat CO₂ is in a supercritical state.

In some embodiments, the thermal energy storage fluid has a heatcapacity of at least about 0.1 Kilojoules (KJ)/Kg*K, 1 KJ/Kg*K, 2KJ/Kg*K, 3 KJ/Kg*K, 4 KJ/Kg*K, 5 KJ/Kg*K, 6 KJ/Kg*K, 7 KJ/Kg*K, 8KJ/Kg*K, 9 KJ/Kg*K, 10 KJ/Kg*K, 15 KJ/Kg*K, 20 KJ/Kg*K, 25 KJ/Kg*K, 30KJ/Kg*K, 40 KJ/Kg*K, or 50 KJ/Kg*K. The thermal energy storage fluid mayhave a heat capacity from about 1 KJ/Kg*K and 100 KJ/Kg*K, or 3 KJ/Kg*Kand 50 KJ/Kg*K.

In some embodiments, the proportion of carbon dioxide to the at leastone disorder-inducing species is at least about 0.1:1, or at least about0.5:1, or at least about 1:1, or at least about 2:1, or at least about3:1, or at least about 4:1, or at least about 5:1, or at least about10:1, or at least about 20:1, or at least about 30:1, or at least about40:1, or at least about 50:1, or at least about 100:1.

Supercritical fluid mixtures of the invention are based on theunexpected realization that heat storage properties of fluids, such asheat capacities, may be increased by inducing inhomogeneities at orabove the critical point (see FIG. 3). With reference to the phasediagram of FIG. 3, a supercritical fluid may experience an improved heatcapacity in the supercritical portion of the phase diagram of the fluidalong, for example, a “ridge” extending from the critical point,separating “liquid-like” and “vapor-like” region. Such improved heatcapacity may be experienced along other paths in the critical region.With reference to FIG. 4, for fluid mixtures the properties of the fluidmixture extending away from a critical line to a plane may be adjustedto enhance the heat capacity of the fluid mixture. In some embodiments,the composition of a fluid mixture comprising carbon dioxide and one ormore disorder-inducing species may be selected to enhance the heatcapacity of the fluid mixture. This may provide increases in intrinsicheat capacity of the fluid mixture in the vicinity of the critical point(line), and provide extended enhancement of the heat capacity into thesupercritical region.

In some situations, carbon dioxide is mixed with a disorder-inducingspecies to form a fluid mixture, and the pressure and/or temperature ofthe fluid mixture is increased to a point such that the fluid mixture isin a supercritical regime. In some cases, this is achieved via atrans-critical process with fluid properties (e.g., pressure,temperature, composition) selected to bring the fluid to supercriticalconditions. In some cases, this is above a critical point (temperature,pressure) of the fluid mixture.

In some embodiments, the thermal energy storage fluid has adisorder-inducing species at a composition (mole %) between 0.1% and70%, or between about 1% and 60%, or between about 5% and 50%. In somesituations, the thermal energy storage fluid has a disorder-inducingspecies at a composition of at least about 1%, 2%, 3%, 4%, 5%, 6%, 7%,8%, 9%, 10%, 15%, 20%, 30%, 40%, 50%, or 60%. The balance of the fluidmixture is CO₂. In an example, a supercritical fluid mixture comprisesabout 60% CO₂ and 40% ethanol.

As an example, a fluid mixture comprises 80% CO₂ and 20% ethanol. Asanother example, a fluid mixture comprises 90% CO₂ and 10% nitromethane.As another example, a fluid mixture comprises 95% CO₂ and 5% aceticacid.

In some embodiments, the thermal energy storage fluid has two or moredisorder-inducing species at a composition (mole %) between 0.1% and70%, or between about 1% and 60%, or between about 5% and 50%. In somesituations, the thermal energy storage fluid has two or moredisorder-inducing species at a composition of at least about 1%, 2%, 3%,4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 30%, 40%, 50%, or 60%. Thebalance of the fluid mixture is CO₂. In an example, a supercriticalfluid mixture comprises about 60% CO₂ and 20% methanol and 20% ethanol.

In some embodiments, the disorder-inducing species comprises one or moreorganic compounds. An organic compound can be selected from alkanes,alkenes, alkynes, alcohols, carboxylic acids, ketones, aldehydes, orcombinations thereof. In some situations, the disorder-inducing speciescomprises one or more compounds selected from Table 1.

TABLE 1 disorder-inducing species. Compound Pc (MPa) Tc (deg C.)Nitromethane 5.9 315 Acetic acid 5.8 320 Acetone 4.8 325 Methanol 8.1240 Ethanol 6.1 241 Isopropanol 4.9 235 Glycerol 7.5 577 Ethyl acetate3.8 257 Isobutyl acetate 3 287 Dimethyl carbonate 4.8 284 Dimethyl ether5.3 127 Perfluorobutane 2.3 113 R32 (difluoromethane) 5.8 78 Methane 4.6−82 Cyclohexane 4.1 281 Toluene 4.1 319 2-Methyl Propylcyclohexane 3.1386 Dodecane 1.8 385 Tetra hydrofuran 5.2 267 Water 22 374 Pc = criticalpressure; Tc = critical temperature.

A fluid mixture comprising carbon dioxide and one or moredisorder-inducing species can include one or more, two or more, three ormore, four or more, five or more, or six or more disorder-inducingspecies. For instance, a fluid mixture can comprise carbon dioxide andethanol, or carbon dioxide, ethanol and propanol (e.g., isopropanol).

In some embodiments, anomalously high heat capacities are achieved at orabove the critical point, which may arise from extended densityinhomogeneities as may be produced with the aid of disorder-inducingspecies. The length scale is characterized by a correlation length (ξ)much greater than the molecular scale and is due to enhanced attractiveinteractions (“clustering”) between molecules near the critical point.Maximization of heat capacity and its persistence in some cases isachieved by maximizing disorder in fluid systems.

The heat capacity of a fluid mixture comprising CO2 and one or moreother components may be continuously tuned by varying the fluid mixturecomposition. In some cases, this may provide at least a 2, 3, 4, 5, 6,7, 8, 9 or 10-fold increase in heat capacity at the critical point withrespect to the pure components, and a significant enhancement of heatcapacity in the supercritical regime and away from the critical point.

Such increases in heat capacity have been achieved in a fluid mixturehaving nitromethane and isobutanol, as discussed in Losada-Pérez, P, G.Pérez-Sánchez, J. Troncoso, and C. A. Cerdeiriña, J. Chem. Phys, 132,154509 (2010), which is entirely incorporated herein by reference. FIG.5 is an exemplary plot of heat capacity (Cp) as a function oftemperature (T) and component composition (X) for a fluid mixturecomprising nitromethane and isobutanol. FIG. 6 is a cross-section of theplot of FIG. 5 taken at 291 K. FIGS. 5 and 6 show a substantial increasein Cp at a temperature of about 291 Kelvin (K) and a fluid mixturecomposition of about 45 mole % nitromethane.

Power Generation Systems

Another aspect of the invention provides a circulatory fluid flow systemconfigured to store energy and/or generate power. In some embodiments,the system employs a fluid mixture comprising a supercritical fluid anda disorder-inducing species. The fluid mixture is both a thermal energystorage fluid and a working fluid. The supercritical fluid in some casesis CO₂. In other embodiments, the system employs a fluid mixturecomprising a supercritical fluid and a disorder-inducing species. Thefluid mixture in such a case is the thermal energy storage material. Thesystem employs a separate working fluid, which may be a supercriticalfluid.

In some cases, a supercritical fluid is the working fluid of a powergeneration system. In other cases, the supercritical fluid is thethermal energy storage medium of a power generation system.

FIG. 7 shows a system 700 configured to generate power, in accordancewith an embodiment of the invention. The system 700 comprises acompressor 701, heat exchanger 702 and power generator 703. The powergenerator 702 may be a turbine adapted to generate electricity, such as,for example, via electromagnetic induction. In some situations, thesystem 700 includes an energy source 704, such as a renewable energysource. The system 700 is a circulatory fluid flow system in which aworking fluid comprising a fluid mixture having a supercritical fluid,such as CO₂, and one or more secondary components, such as adisorder-inducing species, is directed through a fluid flow path havingthe compressor 701, heat exchanger 702 and power generator 703. Thedirection of fluid flow in the circulatory fluid flow system isindicated by arrows between the unit operations of the system 700.

The renewable energy source can be configured to direct thermal energyto the heat exchanger 702 or a fluid in fluid communication with theheat exchange 702. For example, the renewable energy source 704 can be asolar concentrator that collects solar radiation and directs thermalenergy collected from collected solar radiation to the heat exchanger702, either directly, such as with the aid of optics, or with the aid ofa secondary fluid or thermal energy storage material, such as, forexample, molten salt or a supercritical fluid adapted to store thermalenergy. Solar concentrators are described in, for example,WO/2011/116141 to Joseph et al., which is entirely incorporated hereinby reference.

In an exemplary operation of the system 700, the working fluid iscompressed by the compressor 701 and directed into the heat exchanger702, where heat is applied to the working fluid. The compressor 701compresses the working fluid to a pressure at or above the criticalpressure of the working fluid, and the heat exchanger 702 supplies heatto the working fluid, which may increase the temperature of the workingfluid to a temperature at or above the critical temperature of theworking fluid. Next, the working fluid is directed to the powergenerator 703, which generates electricity. In some situations, heat isremoved from the working fluid, such as in the power generator 703and/or another heat exchanger between the power generator 703 and thecompressor 701.

In some embodiments, the working fluid is operated at or above acritical point of the working fluid from the point the system 700 isinitiated. In such a case, the system 700 can initiate with the workingfluid in a supercritical regime of the working fluid. The compressorand/or heat exchanger may be used to increase the pressure andtemperature of the working fluid from a first point (P₁, T₁) in thesupercritical regime to a second point (P₂, T₂) in the supercriticalregime.

The system 700 may be adapted to operate under the Brayton cycle. FIGS.8A and 8B show pressure-volume and temperature-entropy diagrams for asystem operating under a Brayton cycle, in accordance with an embodimentof the invention. The system of FIGS. 8A and 8B operates with the aid ofa working fluid comprising a supercritical fluid mixture. Thesupercritical fluid mixture in some cases includes CO₂ and one or moresecondary fluids, such as one or more disorder-inducing species.

From step 1 to step 2, the working fluid is compressed, such as with theaid of a compressor. Next, from step 2 to step 3, heat is added to thecompressed working fluid. In some cases, heat is added to the workingfluid with the aid of a heat exchanger in thermal communication with theworking fluid. Next, from step 3 to step 4, the working fluid isdirected to a power generator, which is used to generate electricityfrom the working fluid. From step 3 to step 4, the working fluidundergoes expansion and a reduction in pressure. From step 4 to step 1,heat is rejected from the working fluid. Heat can be removed from theworking fluid with the aid of a heat exchanger. Heat rejected from theworking fluid can be directed to a secondary fluid, such as water togenerate steam. The secondary fluid may be a supercritical fluid, suchas supercritical CO₂ and, in some cases, a disorder-inducing species.

In some embodiments, from step 2 to step 3, heat can be added to theworking fluid (e.g., supercritical fluid) from an energy source, such asheat generated from combustion (e.g., fuel or coal combustion), anuclear reactor or a solar concentrator. In some situations, heat fromthe energy source may be stored in a thermal energy storage material,which may be brought in thermal communication with the working fluid totransfer heat from the thermal energy storage material to the workingfluid. In some cases, the thermal energy storage material is brought indirect contact with the working fluid to transfer energy.

With continued reference to FIGS. 8A and 8B, energy is introduced to theworking fluid in the form of heat from step 2 to step 3. In addition,energy may be introduced to the working fluid in the form of work uponcompression from step 1 to step 2.

Heat (energy) introduced into the system is represented asQ₂₋₃=C_(p)(T₃−T₂), where system enthalpy is typically increased viatemperature increase. An increase in C_(p) by a factor of 10 caneffectuate an increase in the amount of energy stored in the system byabout the same factor without an increase in temperature. This avoidsoperation at higher temperature and some of the associated materials andmaintenance issues. Heat loss may decline as the thermal gradient to theambient is significantly reduced. Efficiency may be improved withreduced containment insulation. Overall cycle efficiency can alsodirectly improve as input enthalpy is increased.

In an exemplary implementation of the cycle of FIGS. 8A and 8B, aworking fluid comprising a supercritical fluid mixture having CO₂ andone or more secondary species (or components), such as one or moredisorder-inducing species, is directed through a closed-loop cyclecomprising a compressor, heat exchanger in thermal communication with asource of energy, and a turbine. From step 1 to step 2, the pressure ofthe working fluid is increased with the aid of the compressor. Next,from step 2 to step 3, the working fluid is directed to the heatexchanger that is in thermal communication with the source of energy,such as a solar concentrator for collecting and concentrating solarradiation, a source of combustion, or nuclear reactor. In the heatexchanger, heat is added to the working fluid. Next, from step 3 to step4, the working fluid is directed to the power generator, such as aturbine, to generate electricity. From step 4 to step 1, the workingfluid is directed from the power generator to the compressor, and heatis removed from the working fluid.

Supercritical fluid mixtures provided herein can be tailored to decreasethe cost of systems operating under the Brayton cycle. For example, asupercritical working fluid with a high heat capacity can decrease theoperating pressure and/or temperature of the system, leading to savingsin materials and maintenance costs and expenses.

FIG. 9 shows a system 900 comprising a renewable energy source 901, acompressor 902, a first fluid storage vessel 903, a second fluid storagevessel 904, a power generator 905, an electricity (or power) grid 906and an electrical load 907, in accordance with an embodiment of theinvention. The renewable energy source 901 can include one or more solarconcentrators, wind turbines, wave generators, or geothermal energysources.

In some cases, the system 900 can include a heat exchanger between thecompressor 902 and the second fluid storage vessel 904. The heatexchanger can be configured to direct energy to the working fluid. Theenergy can be provided by way of an energy source, such as a renewableenergy source (e.g., solar concentrator).

In some embodiments, the compressor 902, first fluid storage vessel 903,second fluid storage vessel 904 and power generator 905 define aclose-loop power generation cycle. The cycle in some cases can operateunder the Brayton cycle (see below). The cycle can operate with the aidof a working fluid that comprises a primary fluid and one or moresecondary fluids. In some embodiments, the working fluid is asupercritical fluid mixture that comprises CO₂ as the primary fluid andone or more secondary fluids, such as one or more disorder-inducingspecies (e.g., ethanol, isopropanol), as described elsewhere herein.

In an exemplary implementation of the system 900, the renewable energysource 901 generates electricity, which may be directed to theelectricity grid 906 and subsequently directed to the electrical load907. In some cases, electricity from the renewable energy source is usedto power the compressor 902, which compresses a fluid from the firstfluid storage vessel 903 and directs the compressed fluid to the secondfluid storage vessel 904. In an example, the renewable energy source 901is used to compress the fluid from the first storage vessel 903 to thesecond storage vessel 904 in off-peak (or low electricity demand)conditions. In some embodiments, when energy is required, such as inon-peak (or high electricity demand) conditions, the compressed fluidfrom the second fluid storage vessel 904 is directed to the powergenerator 905, which generates electricity. Electricity generated by thepower generator 905 may be directed to the power grid 906 andsubsequently directed to the electrical load 907.

In some embodiments, a circulatory fluid flow system is provided havinga fluid mixture comprising a supercritical fluid and a disorder-inducingspecies. The fluid mixture in such a case is the thermal energy storagematerial. The system employs a separate working fluid, which may be asupercritical fluid.

FIG. 10 shows a system 1000 configured to generate electricity, inaccordance with an embodiment of the invention. The system 1000 includesa first circulatory fluid flow cycle comprising a compressor 1001, firstheat exchanger 1002 and power generator 1003. A fluid flow path directsa first working fluid from the compressor 1001 to the first heatexchanger, from the first heat exchanger 1002 to the power generator1003, and subsequently from the power generator 1003 to the compressor1001. The first circulatory fluid flow cycle can include another heatexchanger between the power generator 1003 and the compressor 1001,which is configured to remove heat from the first working fluid of thefirst circulatory fluid flow cycle. In some situations, the firstcirculatory fluid flow cycle may operate under the Brayton cycle.

The system 1000 also includes a second circulatory fluid flow cycle thatcomprises the first heat exchanger 1002, a second heat exchanger 1004and pump 1005. A fluid flow path directs a second working fluid of thesecond circulatory fluid flow cycle from the first heat exchanger 1002to the second heat exchanger 1004, and subsequently from the second heatexchanger 1004 to the pump 1005. Flow of the second working fluidthrough the second circulatory fluid flow path can be facilitated withthe aid of the pump 1005. Together, the first circulatory fluid flowcycle and the second circulatory fluid flow cycle can operate under theRankine cycle.

The system 1000 in some cases includes an energy source 1006 configuredto supply energy to the second heat exchanger 1006. The energy source1006 can be a combustion furnace, nuclear reactor or solar concentratoror other source of energy. In some embodiments, the energy source 1006is configured to supply thermal energy to the second heat exchanger1002, which is subsequently directed to the second working fluid of thesecond circulatory fluid flow cycle.

In some embodiments, the first working fluid is water, a hydrocarbon orother organic species. In some situations, the first working fluid is analkane or alkene, such as n-pentane or toluene. In some embodiments, thefirst working fluid is a supercritical fluid.

In some embodiments, the second working fluid is a supercritical fluidmixture. In an example, the supercritical fluid mixture includessupercritical CO₂ and one or more secondary components, such as one ormore disorder-inducing species. The properties of the second workingfluid in some cases are selected to optimize the heat capacity of thesecond working fluid.

In an exemplary operation of the system 1000, the second working fluidof the second circulatory fluid flow cycle is heated in the second heatexchanger 1004 with the aid of thermal or electrical energy provided bythe energy source 1006 and directed to the first heat exchanger 1002with the aid of the pump 1005. In the first heat exchanger 1002, energyis transferred from the second working fluid to the first working fluidof the first circulatory fluid flow cycle. The first working fluid isthen directed to the power generator 1003, which generates power fromthe first working fluid. The first working fluid is then directed to thecompressor 1001, which compresses the first working fluid and directsthe first working fluid to the first heat exchanger 1002.

Thermal Energy Storage Systems

Another aspect of the invention provides a system for storing thermalenergy. The system comprises a supercritical fluid. In some embodiments,the supercritical fluid is a fluid mixture comprising carbon dioxide andone or more disorder-inducing species. The disorder-inducing species canbe as described elsewhere herein.

FIG. 11 shows a system 1100 for storing thermal energy, in accordancewith an embodiment of the invention. The system 1100 includes a firstvessel 1101 in thermal communication with a source of energy 1102. Thesource of energy 1102 may be a combustion system, nuclear reactor orrenewable energy source (e.g., solar concentrator). A pump 1103 directsa working fluid from the first vessel 1101 to a second vessel 1104. Insome embodiments, the working fluid is a supercritical fluid comprisingcarbon dioxide and one or more disorder-inducing species. Thedisorder-inducing species can be as described elsewhere herein. The pumpcan select the pressure of the working fluid to be at or above thecritical pressure of the working fluid. The temperature of the workingfluid can be selected to be at or above the critical temperature withthe aid of the source of energy 1102.

The first vessel 1101 and/or the second vessel 1104 can be a storagevessel for storing the working fluid for later use. In some cases, thefirst vessel 1101 and/or the second vessel 1104 can be detachable from afluid flow path having the first vessel 1101, the pump 1103 and thesecond vessel 1104. This may permit energy to be transferred to theworking fluid, which may be transported to another location (e.g.,remote location) for use.

In some situations, one or both of the first vessel 1101 and the secondvessel 1104 is a heat exchanger adapted to facilitate the transfer ofenergy to the working fluid or from the working fluid to another fluid,such as a secondary fluid (e.g., water).

Systems and methods provided herein can be implemented with the aid of acontroller having a processor (e.g., central processing unit) thatexecutes machine-executable instructions, which may be located on amemory location (e.g., read-only memory, random-access memory, flashmemory) of the controller. The controller can be a feedback controller.The controller can regulate various system parameters, such as systempressure, temperature, and flow rate. The controller may be programmedto monitor fluid pressure and/or temperature and maintain system and/orfluid parameters to achieve a given system performance. For instance,the controller can be programmed to maintain system pressure and/ortemperature to maximize the heat capacity of the thermal energy storagemedium or working fluid.

EXAMPLE 1

A supercritical fluid mixture comprises carbon dioxide CO₂ and ethanolin a composition (mole %) of about 90% CO₂ and 10% ethanol. Thecomposition is selected to provide a heat capacity of the fluid mixturethat may be suitable for use with thermal energy storage.

EXAMPLE 2

A system, such as the system 700 of FIG. 7, is used to generate power.The working fluid is a fluid mixture comprising Supercritical CO₂ andethanol at a composition (mole %) of about 90% CO₂ and 10% ethanol. Thetemperature of the working fluid is about 305 K. In a first step, thesupercritical working fluid is directed to a heat exchanger, whichsupplies heat to the working fluid. In a second step, the working fluidis directed to a turbine to generate power. Next, in a third step, heatis removed from the working fluid, in some cases with the aid of anotherheat exchanger. The working fluid is then returned to the compressor.

It should be understood from the foregoing that, while particularimplementations have been illustrated and described, variousmodifications may be made thereto and are contemplated herein. It isalso not intended that the invention be limited by the specific examplesprovided within the specification. While the invention has beendescribed with reference to the aforementioned specification, thedescriptions and illustrations of embodiments of the invention hereinare not meant to be construed in a limiting sense. Furthermore, it shallbe understood that all aspects of the invention are not limited to thespecific depictions, configurations or relative proportions set forthherein which depend upon a variety of conditions and variables. Variousmodifications in form and detail of the embodiments of the inventionwill be apparent to a person skilled in the art. It is thereforecontemplated that the invention shall also cover any such modifications,variations and equivalents.

What is claimed is:
 1. A system, comprising: a circulatory fluid flowpath having: (a) a compressor configured to accept a working fluid andincrease the pressure of the working fluid, wherein said working fluidis a supercritical fluid mixture comprising carbon dioxide and at leastone disorder-inducing species; (b) a heat exchanger downstream of thecompressor, said heat exchanger adapted to accept said working fluidfrom said compressor and provide heat to said working fluid; and (c) apower generator downstream of the heat exchanger, said power generatoradapted to generate power upon the flow of said working fluid throughsaid power generator, and direct said working fluid to said compressor,wherein said system: (i) circulates said working fluid along saidcirculatory fluid flow path, and (ii) increases the pressure and/ortemperature of said working fluid to above the critical pressure andcritical temperature of said working fluid.
 2. The system of claim 1,further comprising an another circulatory fluid flow path forcirculating an another working fluid, said another fluid flow pathcomprising: (a) an another heat exchanger for providing energy to saidanother working fluid; (b) a pump downstream of said another heatexchanger, said pump adapted to accept an another working fluid andincrease the pressure of said another working fluid; and (c) said heatexchanger, wherein said heat exchanger accepts said another workingfluid and transfers energy from said another working fluid to saidworking fluid.
 3. The system of claim 2, further comprising an energysource in thermal communication with said another heat exchanger,wherein said energy source provides energy to said another heatexchanger.
 4. The system of claim 3, wherein said energy source is acombustion system, nuclear reactor or solar concentrator.
 5. The systemof claim 2, wherein said another working fluid comprises a supercriticalfluid mixture comprising carbon dioxide and at least onedisorder-inducing species.
 6. The system of claim 1, further comprising(d) an another heat exchanger downstream of said power generator, saidanother heat exchanger adapted to (i) accept said working fluid fromsaid power generator, (ii) remove heat from said working fluid and (iii)direct said working fluid to said compressor.
 7. The system of claim 1,wherein said at least one disorder-inducing species is selected from thegroup consisting of an alkane, alkene, alcohol, aldehyde, ketone, ether,ester, water, fluorinated hydrocarbons, nitromethane, aromatichydrocarbons and carboxylic acid.
 8. The system of claim 1, wherein saidcompressor increases the pressure of the working fluid at or above acritical pressure of the working fluid.
 9. The system of claim 1,wherein said heat exchanger increases the temperature of the workingfluid at or above a critical temperature of the working fluid.
 10. Asystem, comprising: a circulatory fluid flow path having: (a) a firstheat exchanger adapted to accept a working fluid and provide energy tosaid working fluid, wherein said working fluid is a supercritical fluidmixture comprising carbon dioxide and at least one disorder-inducingspecies; (b) a pump downstream of said first heat exchanger, whereinsaid pump accepts said working fluid from said first heat exchanger andincreases the pressure of said working fluid; and (c) a second heatexchanger downstream of said pump, said second heat exchanger adapted toaccept said working fluid from said pump and transfer energy from saidworking fluid to a secondary fluid, wherein said system circulates saidworking fluid along said circulatory fluid flow path.
 11. The system ofclaim 10, further comprising an energy source in thermal communicationwith said first heat exchanger, wherein said energy source providesenergy to said first heat exchanger.
 12. The system of claim 11, whereinsaid energy source is a combustion system, nuclear reactor of solarconcentrator.
 13. The system of claim 10, wherein said system maintainsthe pressure and temperature of said working fluid at a supercriticalpressure and supercritical temperature, respectively, of said workingfluid.
 14. The system of claim 10, wherein said at least onedisorder-inducing species is selected from the group consisting of analkane, alkene, alcohol, aldehyde, ketone, ether, ester, water,fluorinated hydrocarbons, nitromethane, aromatic hydrocarbons andcarboxylic acid.
 15. A thermal energy storage fluid, comprising carbondioxide and at least one disorder-inducing species, wherein theproportion of carbon dioxide to said at least one disorder-inducingspecies in said thermal energy storage fluid is sufficient to inducedisorder in said thermal energy storage fluid at or above a criticalpoint of said thermal energy storage fluid.
 16. The thermal energystorage fluid of claim 15, wherein said at least one disorder-inducingspecies is selected from the group consisting of an alkane, alkene,alcohol, aldehyde, ketone, ether, ester, water, fluorinatedhydrocarbons, nitromethane, aromatic hydrocarbons and carboxylic acid.17. The thermal energy storage fluid of claim 15, wherein said thermalenergy storage fluid has a heat capacity of at least about 25 KJ/Kg*K.18. The thermal energy storage fluid of claim 15, wherein saidproportion is at least about 0.5:1.
 19. The thermal energy storage fluidof claim 15, wherein said proportion is at least about 1:1.
 20. Thethermal energy storage fluid of claim 15, wherein said proportion is atleast about 2:1.
 21. A method for forming a thermal energy storagefluid, comprising providing a fluid mixture having carbon dioxide and atleast one disorder-inducing species in a vessel at a proportion ofcarbon dioxide to said at least one disorder-inducing species that isselected to induce disorder in said thermal energy storage fluid at orabove a critical point of said thermal energy storage fluid.
 22. Themethod of claim 21, wherein said at least one disorder-inducing speciesis selected from the group consisting of an alkane, alkene, alcohol,aldehyde, ketone, ether, ester, water, fluorinated hydrocarbons,nitromethane, aromatic hydrocarbons and carboxylic acid.
 23. The methodof claim 21, wherein said thermal energy storage fluid has a heatcapacity of at least about 25 KJ/Kg*K.
 24. The method of claim 21,wherein said proportion is at least about 0.5:1.
 25. The method of claim21, wherein said proportion is at least about 1:1.
 26. The method ofclaim 21, wherein said proportion is at least about 2:1.